Variable wavelength transceiver

ABSTRACT

A variable wavelength transceiver includes a first variable wavelength transmitter, capable of generating an electromagnetic wave of a first wavelength, a second detector of an electromagnetic wave of a second wavelength, and a collision detector coupled to the second detector and the variable wavelength transmitter, the collision detector determining when the energy level of the second detector exceeds a threshold value while the variable wavelength transmitter transmits the electromagnetic wave.

BACKGROUND

This invention relates to optical communication systems.

Local area networks (LANs) connect personal and mini-computers with eachother and with other shared resources, for example, printers, fileservers, and mainframe computers. Separate LANs can be joined viabridges to form larger linked systems of computers and shared resources.

One of the most popular standards for construction and operation of LANsis the IEEE 802.3 Protocol for Local Area Networks, popularly called theEthernet standard. The 802.3 standard prescribes a carrier sensemultiple access with collision detection (CSMA/CD) protocol for mediumaccess control (MAC), where "medium" refers to a communication medium.The 802.3 CSMA/CD standard allows a number of LAN devices to communicatewithin a shared communication medium, without central messagearbitration. Each LAN device, when sending a message, listens to thecommunication medium at the same time. If another LAN device also beginssending a message, the first LAN device can sense the multiple accessand thereby detect a "collision" between the signals of the twomessages. For example, in an Ethernet LAN using coaxial cable as itstransmission medium, and employing a digital baseband method ofsignaling, a collision occurs when the first LAN device starts to send asignal but simultaneously detects a signal coming from a second LANdevice within the transmission medium. The first LAN device declares a"collision", and that device (and any other LAN devices also attemptingto send messages) implements a standard back-off protocol. The back-offprotocol typically requires the LAN device to wait increasingly longerrandom periods of time after each collision before attempting to resendits message.

The 802.3 CSMA/CD standard has also been implemented in non-metallictransmission systems, such as optical fiber and spread-spectrumradiofrequency communications networks. The 802.3 CSMA/CD standard maybe applied to any broadcast medium, so long as the medium can support amethod for collision detection. For coaxial cable media (as above), onecollision detection method measures the energy density at thetransceiver, since signal levels of all transceivers measured anywherealong the transmission line are similar.

Infrared communication techniques are attractive because they offer anumber of benefits, including low transceiver costs, low powerconsumption, high device reliability, high biosafety, low installationcosts, and little required cabling. Existing infrared transmissionstandards (for example, IRDA94) provide for data transmissionpoint-to-point, single access protocols. These are typicallyhalf-duplex, utilize amplitude modulation (AM), and do not provide anyform of collision detection. Instead, these standards typically rely oncollision avoidance. A true multiaccess infrared LAN requires a methodof collision detection for high utilization of available IR bandwidth.Furthermore, a useful method of IR collision detection would allowimplementing the full 802.3 CSMA/CD protocol, permitting an IRmultiaccess LAN to integrate seamlessly with other conventional LANsystems and devices.

SUMMARY

In general, one aspect features apparatus for communication including afirst variable wavelength transmitter, capable of generating anelectromagnetic wave of a first wavelength, a second detector of anelectromagnetic wave of a second wavelength, and a collision detectorcoupled to the second detector and the variable wavelength transmitter,the collision detector determining when the energy level of the seconddetector exceeds a threshold value while the variable wavelengthtransmitter transmits the electromagnetic wave.

Embodiments may include the following features. The first variabletransmitter can generate electromagnetic waves of a plurality ofdiscrete wavelengths. The second detector can detect electromagneticwaves of a plurality of discrete wavelengths. The apparatus can includea second variable wavelength transmitter, capable of generating anelectromagnetic wave of the second wavelength, and a first detector ofan electromagnetic wave of the first wavelength, where the collisiondetector is coupled to the first detector, and where the collisiondetector determines whether the energy level of the first detectorexceeds a threshold during a transmission of an electromagnetic wave ofthe second wavelength by the second variable transmitter.

The apparatus can include a first detector of an electromagnetic wave ofthe first wavelength, where the collision detector is coupled to thefirst detector, and where the collision detector determines whether theenergy level of the first detector exceeds a threshold during atransmission of an electromagnetic wave of the second wavelength by thefirst variable transmitter.

The first variable transmitter can include a first light emitting diodeportion, and a second light emitting diode portion, the first and secondlight emitting diode portions arranged to form an optical cavity betweenthem. The optical cavity can be a Fabry-Perot interferometer. The secondlight emitting diode portion can have an imbedded electrode that canvary the conduction across the second light emitting diode portion. Thesecond detector can include an optical detector, and a thin-filmvariable interferometer layer coupled to the optical detector. Thethin-film variable interferometer layer can further include first andsecond dielectric thin-film layers each having respective conductivecontacts to apply charge to each respective thin-film layer to vary itsdielectric constant. The first and second wavelengths can be in theinfrared portion of the electromagnetic spectrum.

Advantages may include one or more of the following. A variablewavelength transceiver can quickly and easily implement wavelengthdivision multiplexing, allowing an IR LAN to adaptively add more signalcarriers as needed. If certain wavelengths have more ambient noise thanothers, all communications can be simply transferred over to less-noisywavelengths. The multiple wavelengths used in the preferred collisiondetection scheme can be generated by one variable frequency laser diode.

Other features and advantages of the invention will become apparent fromthe following description and from the claims.

DRAWINGS

FIG. 1 is a schematic block diagram of a dual-wavelength transmitter andreceiver.

FIGS. 2a and 2b are cross-sections of multilayer thin film filters.

FIG. 3 is a schematic diagram of two LAN devices in an IR networkenvironment.

FIG. 4 is a signal timing diagram of transmission and detection of IRsignals and collisions.

FIG. 5 is a coverage diagram of an IR network environment.

FIG. 6 is a schematic diagram of a IR repeater.

FIGS. 7 and 8 are coverage diagrams of an IR network environment with IRrepeaters.

FIG. 9 is a schematic diagram of a fiberoptic network.

FIG. 10 is a schematic diagram of a multiple access fiberoptic network.

FIG. 11 is a schematic diagram of a dual-wavelength transceiver for amultiple access fiberoptic network.

FIG. 12 is a schematic diagram of a variable wavelength laser diodetransmitter.

FIG. 13 is a schematic diagram of a variable wavelength filter receiver.

FIG. 14 is a schematic diagram of a prior art laserbased communicationsystem.

FIG. 15 is a schematic diagram of a bidirectional directed lasercommunication system with collision detection.

FIG. 16 is a schematic diagram of a multiple wavelength detection systemwith collision detection.

FIG. 17 is a schematic diagram of an IR LAN.

DESCRIPTION

Referring to FIG. 1, a dual-wavelength infrared link 10 includes adual-wavelength transmitter 12 and a dual-wavelength receiver 14. Aninfrared tansceiver includes a transmitter and a receiver. Thedual-wavelength transmitter includes two transmitters 16a and 16bemitting IR signals at two specified wavelengths λ₁ and λ₂, e.g., lightemitting diode (LED) lasers centered on 850 nm and 950 nm (nanometers)respectively. As an example, one LED can be a typical GaAs incoherentLED operating in the 950 nm region, and the other LED can be a GaAs LEDdoped with an aluminum impurity to shift its emission band to 850 nm.(In the IR electromagnetic domain, wavelength expressed in nanometers isnormally the measurement unit used, but wavelength and frequency arecomplementary terms for noting the particular electromagnetic wave beingused for transmission of signals).

Infrared link 10 receives signals to be sent from an attachment unitinterface (AUI) (not shown). Typically, each LAN device has an AUI thatgoverns transceiving LAN messages using the appropriate LAN protocols.Driver comparator 20 senses drive level transitions from the AUI andselects which of the drivers 18a or 18b should signal next. LEDs 16a and16b are then driven by respective matched drivers 18a and 18b.

For example, if differential Manchester encoding is used, turning on thefirst wavelength λ₁ might represent a high level, and the secondwavelength λ₂ might represent a low level, so that a transmittertransition from one level to another at the beginning of a bit periodsignals a logical "zero", and no transition at the beginning of the bitperiod signals a logical "one". Any scheme of encoding can be used withtransmitter 12, including a simple scheme where a logical "one" isrepresented by pulses of the first wavelength λ₁ and a logical "zero" isrepresented by pulses of the second wavelength λ₂.

Signals 22a and 22b from the two transmitters 16a and 16b enter the IRtransmission medium (e.g., a room) and are detected by thedual-wavelength receiver 14. The dual-wavelength receiver 14 includestwo bandpass thin film IR filters 24a and 24b tuned to the first andsecond wavelengths λ₁ and λ₂ (wavelengths can also refer to wavelengthbands). Detectors 26a and 26b are coupled to respective IR filters 24aand 24b to detect impinging signals of the corresponding respectivewavelengths. Each detector may be, for example, a photo-diode, avalanchediode, or a PIN photo-diode. Signals from detectors 26a and 26b arecompared by a tri-state detector comparator 28, which provides threepossible outputs: high, low, or collision. A collision occurs when bothdetectors register IR signals above a predetermined threshold, typicallyset above the background noise of the environment, but below the lowestsignal strength received at transceiver 10. The lowest signal strengthis typically received from the LAN device farthest from the receiver(because IR signals attenuate with the inverse square of distance).

Filters 24a and 24b can be constructed as conventional multilayer thinfilm bandpass filters (e.g., a succession of quarter-wave layers of twosubstances 30a and 30b having different optical indices), as shown inFIG. 2a, with a detector 26 placed directly underneath. While such aflat filter works effectively for signals that are normal to the surfaceof the filter, the bandpass function degrades significantly for signalsarriving at other angles. One variation, shown in FIG. 2b, includes ahemispherical set of layers 32 deposited onto a hemispherical lens 33set over detector 26. The resulting hemispherical bandpass filter 32allows IR light to impinge from a wide variety of angles withoutimpairing the bandpass function. If some directionality is desired for aparticular detector configuration (for example, if the IR radiation isonly to come from a particular direction), the hemispherical shape ofthe filter can be altered to add or subtract desired directionalityalong any selected axis.

Referring to FIG. 3, two LAN transceivers 10₁ and 10₂ interact in acommon transmission medium 34, for example, the space within a room. Therespective dual-wavelength transmitters 12₁ and 12₂ are concurrentlytransmitting, and the resulting signals 36₁ and 36₂ will strikerespective dual-wavelength receivers 14₁ and 14₂ substantiallyconcurrently.

The timing diagram of FIG. 4 illustrate the operation of one embodimentof the invention when two LAN transceivers (stations 1 and 2) transmittwo separate, different messages at approximately the same time. Station1 sends a first signal 38a on wavelength band λ₁ and a second, inversesignal 38b on wavelength band λ₂. A short time after station 1 begins,station 2 starts sending a first signal 40a on wavelength band λ₁ and asecond, inverse signal 40b on wavelength band λ₂. As shown, when notsending data (at the beginning of signals 40a and 40b) a LAN transceiversends no signals, which is why signal 40b is not exactly the inverse ofsignal 40a at its beginning. The receiver of station 1 receivesresultant combined signals 44 and 46 in the two wavelengths. Sincestation 2 is some distance from station 1, its signal strength drops asthe inverse square of the distance. So a signal pulse from Station 2 ata wavelength may add only a little to the surrounding background atstation 1.

Collisions can be declared when the station 1 receiver detects signalsat an amplitude higher than some threshold value 48 or 50 for thecorresponding wavelength, at a time when Station 1 is not sending asignal at that wavelength. For example, received signals 44 and 46 (atλ₁ and λ₂) shows multiple collisions. Collision 52a is where receivedsignal 44 is higher than threshold value 48, but λ₁ signal was not sent(signal 38a). Similarly, collision 52c occurs when received signal 46 ishigher than threshold value 50, but no λ₂ signal was sent (signal 38b).By comparing the energy level of the signal at the receiver 14 against athreshold value and against the known signal sent by the localtransmitter 12 for each wavelength band λ₁ and λ₂, collisions cancorrectly be detected and declared.

Each LAN device includes a dual-wavelength transceiver 10 having both IRtransmitter 12 and receiver 14. Since each LAN transceiver transmits anIR signal in only one of its wavelength bands λ₁ and λ₂ at a time, theLAN transceiver can detect collisions if it detects a signal at thesecond wavelength band λ₂ when it is transmitting at the firstwavelength band λ₁, and vice versa. This collision detection method canbe termed generally "wavelength shift keying". Since broadcast IRradiation attenuates rapidly with distance, wavelength shift keyingallows the LAN device to detect a collision when receiving a very lowsignal strength signal of another device at one wavelength band when itsown high signal strength signals are broadcast at the other wavelength.Also, even though multiple ambient reflections tend to "smear" outindividual signal pulses, a local transceiver can still detect acolliding signal on one wavelength while sending (and then detecting)such a smeared-out signal on the second wavelength. Further, by usingtwo wavelengths instead of one, the probability of detecting collisionsduring any given cycle doubles to increase collision detectionefficiency.

Referring to FIG. 5, a typical office infrared LAN includes a networkhub 60, and one or more mobile end stations (MES's) 62a and 62b (e.g.,desktop or laptop computers). Due to multipath reflections anddispersions, the range of a high speed infrared transmitter is limitedto a relatively small radius (for example, 5 meters). As shown, thetransmission field 64 of hub 60 can reach MES 62a but not 62b.Similarly, transmission field 66a of MES 62a can reach hub 60 (allowingreturn messages), but transmission field 66b of MES 62b cannot.

Referring to FIG. 6, a two-port IR repeater 70 includes a transceiver 72coupled to a hemispherical turret lens 74 (for receiving andtransmitting IR information from a number of directions), anddirectional link turret 76 that can move in a half-hemisphericaldirection. Control block 78 controls the passage of signals to and fromtransceiver 72 and link turret 76. Battery 80 powers repeater 70, and isreplenished through power from photovoltaic cell 82 (which can bepowered by the energy provided by ambient office light, for example).

Referring to FIG. 7, both near and far MESs 62a and 62b communicate withLAN hub 60, through use of repeaters (REPs) 70a and 70b. Repeater 70a isplaced within the transmission field 64 of hub 60. Its hemisphericalturret lens 74a receives information from hub 60, transmits it to itsrespective directional turret 76a, which then transmits a focusedtransmission 84a to the directional turret 76b of repeater 70b. Byfocussing the link transmissions 84a and 84b, the IR range can beextended, for example up to 50 meters between repeaters. Repeater 70b,in turn, transmits the repeated signal through its hemispherical turretlens 74b. Since MES 62b is within the transmission field 86b of repeater70b, the repeated signal reaches MES 62b.

Information sent back by MES 62b (through transmission field 66b) isreceived by hemispherical turret lens 74b of repeater 70b, transmittedthrough directional turret 76b to directional turret 76a of repeater70a, and then back to hub 60, through hemispherical turret lens 74a(having transmission field 86a). Thus, distant MES 62b can maintaincommunication with hub 60 through self-contained, self-powered repeaters70a and 70b

Referring to FIG. 8, multiple repeaters 70a, 70b, and 70c, etc. can belinked in a chain to carry signals from a hub 60 to a distant location(not shown). Repeater 70a receives a signal from transmission field 64of hub 60, transmits it via focussed transmission field 84a of itsdirectional turret 76a to directional turret 76b of repeater 70b.Repeater 70b, instead of a hemispherical turret lens has a seconddirectional turret 90b that transmits the signal via transmission field84b to repeater 70c, which also has two directional turrets 76c and 90c.Repeater 70c can send the signal to further repeaters down the chain. Insome applications, each turret can be assigned a different wavelength,to avoid interference and to prevent feedback loops from forming out oftransient reflections. By employing cheap, ambient-light-poweredrepeaters, IR LAN networks can be easily installed to allow transmissionof signals around walls, through doorways, and throughout complicatedarchitectural spaces.

Infrared communications networks can also be formed using optical fiberlinks. Optical fiber networks typically have the topology 100 shown inFIG. 9. Each station 102 is serially linked to immediately adjacentstations by a forward circuit 104 and a backward circuit 106. Whenstation 102a sends a message to station 102c, the message passes throughstation 102b. A failure at station 102b can make the network fail. TheFiber Distributed Data Interconnect (FDDI) standard takes this intoaccount and provides each station with bypass circuitry for reroutingmessages along the network circuit, even when the intermediate stationhas lost power, or is otherwise disabled. Such bypasses can be costlyand may not be reliable under all circumstances.

In another topology (not shown), all stations (or nodes) connect througha central switch. If the central switch fails, all messaging within thenetwork ceases. In each case, the links are half-duplex, wherecommunications pass in only one direction.

Referring to FIG. 10, a bidirectional CSMA/CD optical fiber network 112includes a number of network stations 103, each coupled to abi-directional guided optical transport fiber 110 by bi-directionaltransceiver 105.

Referring to FIG. 11, each station 103 couples to transport fiber 110via dual-wavelength fiber transceiver 105. Dual wavelength fibertransceiver 105 includes a dual wavelength transmitter 12 (includingdriver 18 and two laser diode transmitters 16a and 16b), and a dualwavelength receiver 14 (including detectors 26a and 26b with respectivebandpass filters 24 and 24b and receiver/comparator 28). Dual wavelengthfiber transceiver 105 operates substantially the same as the unguidedinfrared link 10 described above.

The radiant energy from transmitters 16a and 16b, and the radiant energyto detectors 26a and 26b, are coupled through a 5-way fiber coupler 114to bidirectional link 108. In turn, bidirectional link 108 couples(through a 3-way coupler 116) to transport fiber 110, which is the"backbone" of the optical fiber network. To function properly, alloptical fiber connections in the network (both within each station, andcoupling each station to transport fiber 110) must be properly matchedto prevent erroneous collision detections due to reflected signals. Bothends of transport fiber 110 require termination, either within a finalstation (not shown) or within an evanescent fiber terminator (notshown).

The fiberoptic network (which can be termed a "guided" network)illustrated by FIGS. 10 and 11 can be employed at a variety ofwavelengths including infrared, visible, and ultraviolet light.Furthermore, if multiple pairs of wavelengths were employed, wavelengthdivision multiplexing (WDM) would allow many CSMA/CD networks to beplaced within the same single transport fiber 110. Switching and/orrouting among such co-located WDM networks could be accomplished by anystation, allowing great architectural flexibility for network designers.

It may be desirable in some circumstances to replace the fixeddual-wavelength transmitters 12 and detectors 14 of FIGS. 1 and 11 withvariable wavelength counterparts: wavelengths could then be changed asneeded, even during message transmission. Referring to FIG. 12, asemiconductor variable wavelength laser 150 is formed as a cleavedcoupled cavity laser. Each diode portion 152a and 152b of variablewavelength laser 150 includes a gold film upper surface 154a and 154b, aheterojunction 156a and 156b, an insulating layer 158a and 158b, and abonding layer 160a and 160b for attaching each portion 152a and 152b toa substrate 162. In an analogue to, for example, Butterworth orChebyshiev-type filters, each layer in the diode acts as a 1/4λtransmission line filter, providing a proper boundary impedance match atits design wavelength. Impedance mismatch (and hence filtering)increases exponentially with larger (or smaller) wavelengths from thedesign wavelength.

Insulating layer 158b, of the right diode portion 152b, has an imbeddedstripe electrode for varying the conduction in diode portion 152b. Uppercontacts 164a and 164b drive diode portions 152a and 152b to conduct, ornot.

During laser operation, left diode portion 152a is driven intoconduction, while right diode portion 152b is varied from no conductionto full conduction. The cleave 166 between left and right diode portions152a and 152b forms a variable wavelength Fabry-Perot interferometer.The coupling between this interferometer and the left diode portion 152a(conducting as a laser diode) allows the selection of the wavelength ofthe laser. Laser light 168 of a selected frequency radiates from theright diode portion 152b.

Referring to FIG. 13, a variable wavelength receiver 170 has an opticaldetector 172 imbedded within an insulating layer 174. An opticalmatching layer 176 couples detector 172 to a thin-film variableinterferometer layer 178. Variable interferometer 178 includes aplurality of dielectric thin-film layers 180 and 182 (two are shown, butmore can be used to increase precision of wavelength selection) havingmetal contacts 184a and 184b. Charge placed on contacts 184a and 184bcan, by capacitive effects, alter the dielectric constants of thin-filmlayers 180 and 182, altering their optical properties, and forming avariable optical filter. Hemispherical matching lens 186 gathers lightfor guiding into variable interferometer 178 (and then to detector 172).If used in an optical fiber environment, the hemispherical matching lenscan be replaced with an appropriate fiber optical matching section forcoupling variable wavelength receiver 170 to the fiber system.

Variable wavelength transmitter 150 and variable wavelength receiver 170can replace transmitters 16 and detectors 24/26 in FIGS. 1 and 11, toform a variable wavelength optical transmission system. Such a systemcould implement CSMA/CD, and further use a number of differentwavelengths, or frequency channels. Each variable detector/transmittercould assist in implementing flexible wavelength division multiplexingby switching as needed to other pairs of wavelength bands. Multipleseparate CSMA/CD networks could thereby be employed over the samecommunications medium. One network could operate at wavelengths λ₁ andλ₂, while another network could operate at wavelengths λ₃ and λ₄. Thesewavelengths choices could be altered dynamically as needed. Further,only one variable wavelength transmitter 150 and receiver 170 would beneeded per LAN station, since both could rapidly switch to differentoperational wavelengths.

Referring to FIG. 14, a prior art directed but unguided opticalcommunications system 200 typically includes a laser 204a and a receiver204b, with the directed communication link borne by laser light passingtypically through lenses 208. Such directed, but unguided communicationlinks have a number of applications, for example: instances where wireand/or fiberoptic cabling would be prohibitively expensive or difficultto install; temporary installations, such as for trade shows or customerdemonstrations; portable communications links for military and emergencyapplications; and backup links to improve facility reliability. As withRF wireless communication links, unguided directed optical links aresubject to high ambient noise in the environment. Unguided directedoptical systems typically reduce noise interference through opticalamplification by lenses and tubing, to reduce the effect of noisesources (for example, a bird flying through the enlarged beam), butincreasing the need for mechanical stability and pointing accuracy. Asin FIG. 14, if the link is bidirectional (as is usually the case), thentwo sets of optics are required, where a first station 210a and secondstation 210b each has a set of laser and receiver 204a and 206a, 206band 204b respectively, along with lenses 208. With separated directedlinks (from laser 204a to receiver 206b, and from laser 204b to receiver206a), collision detection may not be necessary.

Referring to FIG. 15, a combined unguided directed communications system220 has two compact communications stations 222a and 222b formed frommatched sets of (respectively) laser 224a and receiver 226a, and laser224b and receiver 226b. Only a single set of optics 228a and 228b isrequired at each end, reducing manufacturing costs, increasingmechanical stability and reliability, and easing look-anglerequirements. In the combined system, the wavelength shift keyingmethods described above suffice to provide collision detection, if anyis required. That is, lasers 224a and 224b each has paired laser sourcesat two different wavelengths, or a variable wavelength transmitter asdescribed above and shown in FIG. 12.

Since only one wavelength is in use in the unguided directedcommunications system 220 at any one time, an improved receiver can bedevised which attenuates wideband noise such as is typically found inoutdoors environments. Referring to FIG. 16, an improved multiplewavelength receiver 300 has a number of specified wavelength detectors302a through 302n. Signals from each detector 302 can be filtered bylowpass filters 304a through 304n. Signals can also pass directly to acollision detection matrix block 306. Lowpass filters 304 measure andaverage long-term ambient signals, e.g. noise, therefore the decisionand collision thresholds of collision-detection matrix block 306 can beadjusted accordingly. Thus, the receiver adjusts to a wide variety ofambient noise conditions as the environment of the transmission mediumchanges.

Referring to FIG. 17, a fiberoptic LAN system 400 includes a centralizedLAN transceiver station 402 which can be located in an equipment closetor similar location in an office building or the like, and which iscoupled to one or more LAN servers (not shown) to provide full LANnetworking capabilities. LAN transceiver station 402 includes one ormore separate LAN transceivers 404a-404c, each of which can include thecomponents shown in FIG. 11, including a set of transmitters 16a and 16band a set of detectors 26a and 26b. Each LAN transceiver 404a-404c iscoupled to a respective fiberoptic cable 406a-406c. Each fiberopticcable 406 terminates in a respective hemispherical lens 408a-408c thatis properly optically matched to the end of the cable. Each fiberopticcable 406 can be used to provide IR fiberoptic connection to aparticular region, such as an enclosed office, or a segment of ahallway. Individual hemispherical lenses 408a-408c can therebycommunicate with individual respective MESs 62a-62c, located indifferent portions of an office (or other IR LAN environment).

Fiberoptic LAN system 400 provides a relatively easy and flexibleapproach for LAN cabling an arbitrarily organized environment, where theLAN system provides for two-way communication with portable LANstations. Since no electrical cabling is required from LAN transceiverstation 402 to hemispherical lenses 408, LAN system 400 allowsstraightforward, low maintenance cable placement.

Further, LAN transceivers 404 can be fabricated on a single wafersubstrate, allowing for large numbers of IR fiberoptic cables toterminate in a single, compact LAN transceiver station 402. Such a largescale integration approach also can decrease the costs of implementing asizable IR LAN.

The methods and apparatus described above can find profitableapplication in a number of settings. Networks formed of computers,printers, personal digital assistants (PDAs), and modems can be formedby wireless IR communications links. Such networks can provide idealtemporary connections, such as between buildings having wire-based LANsand laptops or PDAs, or for office equipment mobility in rapidlychanging business environments. Individuals can use this wirelesscapability for mobile home-based computing. In addition, wavelengthshift keying, combined with wavelength division multiplexing, cangreatly increase transmission capacity of optical fiber.

The technologies can be employed to adapt home appliances or officeequipment into a richly connected wireless network. Using bidirectionalcommunication, appliances equipped as computer-operated devices canlearn of each other's existence and cooperatively communicate. Forexample, a stereo system which is wirelessly linked to a telephone cango silent when a nearby telephone begins to ring. A "smart" remotecontrol can learn of the existence of all controllable appliances in thehome, and adapt itself to operate each of them, without userintervention. A separate window air conditioner can query a thermostatlocated somewhere else in the home, and modify its output accordingly. Ahome security system, upon detection of an intruder, can turn on allappliances to attract attention.

The methods and apparatus described herein can also be adapted to "SmartCards" and other authentication devices, for ease of use. Bidirectionalidentification cards can be used to allow a client to approach a secureddoor, conduct an authentication transaction during the approach, andunlock the door, without requiring the usual step of pausing before thedoor and placing the card over a reader, for example. Similarly, quicktransactions using an electronic payment or digital cash transfer arepossible, for example, with motorists passing a properly equippedtollbooth, or with a customer and a properly equipped cash register.

In buildings equipped with suitably sized IR LAN networks as describedabove, guests carrying a smart badge, able to communicate with thenetwork, can be quickly and interactively routed to their destination.The system can locate them as they pass from cell to cell and transmitsuitable audio directions to route them correctly. Also, the location ofthe visitor can be monitored for security purposes, and the visitor canbe given exit routes during emergencies. Similarly, museum guests can beprovided interactive guided tours where information is downloaded to asmart badge based upon the location of the guest vis-a-vis particularexhibits or works of art.

In hospitals, patients can be equipped with a transceiver coupled tobiosensors so that medical information is continually sent to a centraltracking system, even as the patient leaves their room. High availablebandwidth, and suitably sized cells, allows for a large number ofbiodata channels, and for location tracking, should such a patientdevelop a sudden condition away from their room. Medical teams can bethen immediately and reliably dispatched to assist. Patients can alsocarry their medical charts "with them" electronically: any physician ornurse equipped with a medical PDA can approach a patient and quicklydownload their chart, allowing quick response to a patient who is awayfrom their room.

A warehouse equipped with an IR LAN with collision detection can useinexpensive transceivers to track crates and packages. Package transitand content information can be contained locally with each such packagein a flash memory, along with handling precautions and hazardousmaterial emergency instructions. When a so-equipped package arrives,information can be downloaded as the package is physically moved fromthe loading dock, and through the plant. Again with suitable LAN cellsizing, the location of the package can be continuously tracked. Also,electronic sensors can be implanted in the package, to monitortemperature, humidity, shocks, etc. If such parameters exceed approvedlimits, the package can broadcast a request for assistance, e.g., to thewarehouse environment system.

Due to the bidirectionality and the typical high bandwidth IR signals ofthe described methods and apparatus, high fidelity digital audio andvideo signals can be transferred among remote, portable transceivers.Information rich data (stock quotes, news, weather reports, and otherinformative broadcasts) can be made available on inexpensivealphanumeric pagers, audio pagers, or portable television receivers.Software updates can be downloaded on routine, periodic bases, not onlyto personal computers, but also to household appliance microcontrollers.

Other useful products enabled by the technology include robust wirelesskeyboards able to control multiple computers (for those having more thanone computer on a desk), or multiple keyboards attached to a singlecomputer (e.g., for training purposes). Every clock and appliance canhave small transceivers built-in to periodically receive time-of-day andsynchronization information from their environment LAN, allowing allclocks in a home or office to be synchronized accurately. For example,VCRs would be precisely synchronized to program recording start and endtimes.

Other embodiments are within the scope of the claims. For example, otherwavelengths or frequencies of electromagnetic radiation can be employedfor message communication, including visible light, ultraviolet light,and radio frequency EM radiation. Any number of different detectors andtransmitters can be employed, and a variety of filtering mechanisms usedto provide at least two different wavelength or frequency choices forcollision detection. Collision detection can occur by either sensingreceived radiation from both the local and remote transmitters, or bynoting received and thresholded radiation in one channel when the localtransmitter knows it is sending radiation in the other channel.

What is claimed is:
 1. Apparatus for communication comprising:a firsttransmitter capable of generating an electromagnetic wave of a firstwavelength; a second transmitter capable of generating electromagneticwave of a second wavelength; a first detector for detectingelectromagnetic waves of said first wavelength; a second detector fordetecting electromagnetic waves of said second wavelength; and acollision detector coupled to the first detector and the seconddetector, the collision detector indicating a collision occurrence whenthe energy levels of both the first detector and the second detectorexceed a threshold value.
 2. The apparatus of claim 1 wherein the firsttransmitter generates electromagnetic waves of a plurality of discretewavelengths.
 3. The apparatus of claim 1 wherein the second detectordetects electromagnetic waves of a plurality of discrete wavelengths. 4.The apparatus of claim 1 where the first transmitter comprises:a firstlight emitting diode portion; and a second light emitting diode portion,the first and second light emitting diode portions arranged to form anoptical cavity between them.
 5. The apparatus of claim 4 wherein theoptical cavity is a Fabry-Perot interferometer.
 6. The apparatus ofclaim 4 wherein the second light emitting diode portion has an imbeddedelectrode that can vary the conduction across the second light emittingdiode portion.
 7. The apparatus of claim 1 where the second detectorcomprises:an optical detector; and a thin-film variable interferometerlayer coupled to the optical detector.
 8. The apparatus of claim 7 wherethe thin-film variable interferometer layer further comprises first andsecond dielectric thin-film layers each having respective conductivecontacts to apply charge to each respective thin-film layer to vary itsdielectric constant.
 9. The apparatus of claim 1 where the first andsecond wavelengths are in the infrared portion of the electromagneticspectrum.